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United States Patent |
6,084,251
|
Tamamura
,   et al.
|
July 4, 2000
|
Semiconductor light emitting device with carrier diffusion suppressing
layer
Abstract
Disclosed is a semiconductor light emitting device improved in static
characteristics such as operational current and prolonged in service life.
On an n-type GaAs substrate are sequentially grown an n-type GaAs buffer
layer having a thickness of 0.3 .mu.m; an n-type AlGaInP cladding layer
having a thickness of 1 .mu.m; and an active layer having a MQW structure
of GaInP/AlGaInP. Then, a carrier diffusion suppressing layer having a
thickness of 50 nm is grown on the active layer at a reduced V/III ratio.
On the carrier diffusion suppressing layer are sequentially grown a p-type
AlGaInP cladding layer having a thickness of 1 .mu.m; a p-type GaInP layer
having a thickness of 0.1 .mu.m; and a p-type GaAs current cap layer
having a thickness of 0.3 .mu.m. Then, the p-type AlGaInP cladding layer,
p-type GaInP layer, and p-type GaAs current cap layer are selectively
etched by typically photolithography, to form a mesa structure, and an
n-type GaAs current cap layer is grown to be laminated on both sides of
the mesa structure, to form a semiconductor light emitting device.
Inventors:
|
Tamamura; Koshi (Tokyo, JP);
Kawasumi; Takayuki (Kanagawa, JP);
Hirata; Shoji (Kanagawa, JP)
|
Assignee:
|
Sony Corporation (Tokyo, JP)
|
Appl. No.:
|
014683 |
Filed:
|
January 28, 1998 |
Foreign Application Priority Data
Current U.S. Class: |
257/94; 257/96; 257/E33.027; 372/45.01 |
Intern'l Class: |
H01L 033/00; H01S 003/18; H01S 003/19 |
Field of Search: |
257/200,94,96
372/43,45,46
|
References Cited
Attorney, Agent or Firm: Hill & Simpson
Claims
What is claimed is:
1. A semiconductor light emitting device in which a cladding layer of a
first conducting type, an active layer, and a cladding layer of a second
conducting type are formed on a substrate with said active layer
positioned between said first and second cladding layers, said
semiconductor light emitting device comprising:
a diffusion suppressing layer provided on one side of said active layer;
wherein each of said cladding layers is made from a III-V compound
semiconductor; and
a supply ratio of group V to group III raw materials for growth of said
diffusion suppressing layer is different than a supply ratio of group V to
group III raw materials for growth of said cladding layer on the side of
the active layer where said diffusion suppressing layer is provided.
2. A semiconductor light emitting device according to claim 1, wherein said
diffusion suppressing layer is made from the same material as that of said
cladding layer on the side where said diffusion suppressing layer is
provided.
3. A semiconductor light emitting device according to claim 1, wherein said
diffusion suppressing layer is formed on the side where a p-type cladding
layer is provided, and the supply ratio for raw materials for growth of
said diffusion suppressing layer is smaller than the supply ratio for raw
materials for growth of said p-type cladding layer.
4. A semiconductor light emitting device according to claim 1, wherein said
diffusion suppressing layer is formed on the side where an n-type cladding
layer is provided, and the supply ratio for raw materials for growth of
said diffusion suppressing layer is smaller than the supply ratio for raw
materials for growth of said n-type cladding layer.
5. A semiconductor light emitting device according to claim 1, wherein said
diffusion suppressing layer is undoped.
6. A semiconductor light emitting device according to claim 1, wherein a
concentration of carriers in said diffusion suppressing layer is smaller
than a concentration of carriers in said cladding layer positioned on the
same side of said active layer where said diffusion suppressing layer is
provided.
7. A semiconductor light emitting device according to claim 1, wherein said
diffusion suppressing layer is provided adjacently to said active layer.
8. A semiconductor light emitting device according to claim 1, wherein said
diffusion suppressing layer is separated from said active layer.
9. A semiconductor light emitting device according to claim 8, wherein the
distance between said diffusion suppressing layer and said active layer is
30 nm.
10. A semiconductor light emitting device accordingly to claim 1, wherein
said diffusion suppressing layer has a multi-layer structure of layers
grown with a supply ratio different than that for growth of said cladding
layer on the side where said diffusion suppressing layer is provided and
alternate layers of the multi-layer structure are grown with the same
supply ratio as that for growth of said cladding layer.
11. A semiconductor light emitting device comprising:
a first cladding layer of a first conductivity type;
a second cladding layer of a second conductivity type opposite that of the
first cladding layer;
an active layer between said first and second cladding layers;
a carrier diffusion suppressing layer between said active layer and said
second cladding layer, said second cladding layer and said carrier
diffusion suppressing layer having vacancies into which second
conductivity type carriers are to be diffused, said carrier diffusion
layer having smaller vacancies than said second cladding layer;
wherein,
said carrier diffusion suppressing layer is made of a III-V semiconductor
compound whose group III and group V materials are supplied in a first
ratio of group V to group III materials,
said second cladding layer is made of a III-V semiconductor compound whose
group III and group V materials are supplied in a second ratio of group V
to group III materials, said second ratio being different than said first
ratio.
12. The semiconductor light emitting device of claim 11, wherein said group
III and group V materials are the same for the second cladding layer and
the carrier diffusion suppressing layer.
13. The semiconductor light emitting device of claim 11, wherein the second
ratio is greater than the first ratio.
14. The semiconductor light emitting device of claim 11, wherein the second
cladding layer and said carrier diffusion suppressing layer are p-type
layers and the second ratio is greater than the first ratio.
15. The semiconductor light emitting device of claim 11 wherein the carrier
diffusion suppressing layer is not doped.
16. The semiconductor light emitting device of claim 11 wherein the carrier
diffusion suppressing layer is adjacent the active layer.
17. The semiconductor light emitting device of claim 11 wherein said
carrier diffusion suppressing layer is a superlattice structure layer
comprising alternately laminated first and second layers, the first layers
characterized by group III and group V materials supplied in said first
ratio, the second layers characterized by group III and group V materials
supplied in said second ratio.
18. The semiconductor light emitting device of claim 11 wherein the second
cladding layer has a mesa structure and current block layers on opposite
sides of the mesa structure.
19. The semiconductor light emitting device of claim 11 wherein the second
cladding layer and said carrier diffusion suppressing layer are n-type
layers.
20. The semiconductor light emitting device of claim 11 wherein a
concentration of second conductivity type carriers in the carrier
diffusion suppressing layer is smaller than a concentration of second
conductivity type carriers in the second cladding layer.
21. The semiconductor light emitting device of claim 11 wherein the carrier
diffusion suppressing layer and active layer are spaced apart from each
other by a distance about 30 nm.
22. A semiconductor light emitting device, comprising:
a first AlGaInP cladding layer of a first conductivity type;
a second AlGaInP cladding layer of a second conductivity type opposite that
of the first cladding layer;
a multi quantum well GaInP/AlGaInP active layer between said first and
second cladding layers;
a carrier diffusion suppressing layer between said active layer and said
second cladding layer, said carrier diffusion suppressing layer having
vacancies of atoms of a group III element into which carriers are to be
diffused smaller than those of said second cladding layer, said carrier
diffusion suppressing layer being an AlGaInP layer of said second
conductivity type;
wherein,
said carrier diffusion suppressing layer is made of group III and group V
materials supplied in a first ratio of group V to group III materials,
said second cladding layer is made of said group III and group V materials
supplied in a second ratio of group V to group III materials, said second
ratio being different than said first ratio.
23. The semiconductor light emitting device of claim 22 wherein said first
cladding layer is an n-type layer and said second cladding and carrier
diffusion suppressing layers are p-type layers.
24. The semiconductor light emitting device of claim 22 wherein said first
cladding layer is a p-type layer and said second cladding and carrier
diffusion suppressing layers are n-type layers.
25. The semiconductor light emitting device of claim 22 wherein said
carrier diffusion suppressing and second cladding layers are adjacent to
each other.
26. The semiconductor light emitting device of claim 22 wherein said
carrier diffusion suppressing layer and saidactive layer are spaced apart
from each other by a distance of about 30 nm.
27. The semiconductor light emitting device of claim 22 wherein said first
ratio is smaller than said second ratio.
28. The semiconductor light emitting device of claim 22 wherein said
carrier diffusion suppressing layer is a superlattice structure layer.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an AlGaInP based semiconductor light
emitting device having a structure in which a first cladding layer, an
active layer, and a second cladding layer are sequentially laminated.
With respect to AlGaInP based semiconductor light emitting devices
represented by an AlGaInP based laser diode (LD) or light emitting diode
(LED), recently, those each having a wavelength band centered at a
wavelength shorter than 680 nm, typically, at 650 nm or 635 nm have come
to be practically used.
As one example of the above semiconductor light emitting devices, a III-V
compound based semiconductor light emitting device, particularly, an
AlGaInP based semiconductor laser will be described with reference to FIG.
3.
FIG. 3 is a sectional view of a semiconductor laser 120 which is one
example of a related art semiconductor light emitting device. The
semiconductor laser is fabricated by a MOCVD (Metal Organic Chemical Vapor
Deposition) process. Referring to FIG. 3, on a substrate 1 of a first
conducting type, for example, on an n-type GaAs substrate 1 are
sequentially grown an n-type (first conducting type) GaAs buffer layer 2
having a thickness of 0.3 .mu.m; an n-type AlGaInP cladding layer 3 having
a thickness of 1 .mu.m; an active layer 4 having a MQW (Multi-Quantum
Well) structure of GaInP/AlGaInP; a p-type (second conducting type)
AlGaInP cladding layer 5 having a thickness of 1 .mu.m; a p-type GaInP
layer 6 having a thickness of 0.1 .mu.m; and a p-type GaAs current cap
layer 7 having a thickness of 0.3 .mu.m.
The p-type AlGaInP cladding layer 5, p-type GaInP layer 6, and p-type GaAs
current cap layer 7 are then selectively etched by typically
photolithography, to form a mesa structure. Thereafter, an n-type GaAs
current block layer 8 is grown to be laminated on both sides of the mesa
structure, to form a semiconductor light emitting device 120.
With the recent extension of service environments of optical disks and the
like, the semiconductor light emitting device 120 used as light sources of
these optical disks and the like has been required to be improved in
temperature characteristic, particularly, to be prolonged in service life
under output operation of, for example, 30 mW at 80.degree. C.
To meet the above requirement, there have been proposed a method of
increasing a composition ratio of Al in the p-type AlGaInP cladding layer
5 for strengthening confinement of carriers and light in the active layer
4, and a method of laminating a multi-layer thin film structure called a
MQB (Multi-Quantum Barrier) at a portion adjacent to the active layer 4.
Japanese Patent Laid-open No. Hei 6-237038 has proposed a method of
suppressing diffusion of p-type carriers by provision of a multi-layer
film structure in which AlGaInP layers having strain and being different
in composition ratio of Al are laminated.
Further, it may be considered that a Fermi level is raised by increasing a
concentration of carriers in the p-type cladding layer, to thus
substantially strengthen the confinement effect of the active layer 4.
The above-described methods, however, have disadvantages. The growth of the
MQB or the AlGaInP multi-layer film having strain needs strict setting of
a film thickness and a composition ratio, and if actual film thickness and
composition ratio are offset from the setting values upon growth of the
above MQB or the AlGaInP multi-layer film, characteristics of the device
is rather degraded. Also, while it is easy to increase the concentration
of carriers in the p-type cladding layer, if the carriers are diffused up
to the active layer 4, there may occur defects acting as dark defects or
luminescence centers in the active layer, as a result of which static
characteristics such as operational current of the semiconductor light
emitting device 120 are degraded and also the service life is shortened by
progress of diffusion of the carriers due to heat generation or
current-carrying.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a semiconductor light
emitting device improved in static characteristics such as an operational
current and prolonged in service life, which is fabricated by a method in
which a concentration of carriers in a cladding layer is increased and
diffusion of carriers onto an active layer side is suppressed.
To achieve the above object, according to one aspect of the present
invention, there is provided a semiconductor light emitting device in
which a cladding layer of a first conducting type, an active layer, and a
cladding layer of a second conducting type are formed on a substrate, the
semiconductor light emitting device including: a diffusion suppressing
layer provided on at least one of sides where the cladding layers are
provided; wherein each of said cladding layers is made from a III-V
compound semiconductor and a supply ratio, V/III ratio between raw
materials for growth of the diffusion suppressing layer is different from
the V/III ratio between raw materials for growth of the cladding layer on
the side where the diffusion suppressing layer is provided.
The diffusion suppressing layer may be made from the same material as that
of the cladding layer on the side where the diffusion suppressing layer is
provided.
Preferably, the diffusion suppressing layer is formed on the side where a
p-type cladding layer is provided, and a supply ratio, V/III ratio between
raw materials for growth of the diffusion suppressing layer is smaller
than a V/III ratio between raw materials for growth of the p-type cladding
layer; or the diffusion suppressing layer is formed on the side where an
n-type cladding layer is provided, and a supply ratio, V/III ratio between
raw materials for growth of the diffusion suppressing layer is smaller
than a V/III ratio between raw materials for growth of the n-type cladding
layer.
The diffusion suppressing layer may be undoped. A concentration of carriers
in the diffusion suppressing layer is preferably smaller than a
concentration of carriers in the cladding layers on the side where the
diffusion suppressing layer is provided.
The diffusion suppressing layer may be provided adjacently to the active
layer. The diffusion suppressing layer can be separated from the active
layer. In this case, preferably, the distance between the diffusion
suppressing layer and the active layer is set at 30 nm.
The diffusion suppressing layer, preferably, has a multi-layer structure of
layers grown with a V/III ratio different from that for growth of the
cladding layer on the side where the diffusion suppressing layer is
provided and layers grown with the same V/III ratio as that for growth of
the cladding layer.
The diffusion of carriers may sometimes occur in such a form that
interstitial atoms are diffused, but it may generally occur in such a form
that atoms substituted at lattice points are then substituted at
vacancies. To be more specific, for a III-V compound semiconductor, if
atoms of a group II element are taken as a dopant, the atoms are
substituted at the site of a group III element as an acceptor. In this
case, diffusion of the atoms occurs by substitution of the atoms at
vacancies of the group III element. Similarly, atoms of each of a group VI
or IV element are substituted at the site of the associated V or III
element, and diffusion of the atoms occurs by substitution of the atoms at
vacancies of the associated group V or III element.
In summary, to solve the above-described problems, the semiconductor light
emitting device of the present invention has a structure in which at least
a cladding layer of a first conducting type, an active layer, and a
cladding layer of a second conducting type are formed on a substrate,
wherein a carrier diffusion suppressing layer with smaller vacancies into
which carries are to be substituted is grown on at least one of regions
adjacent to the active layer by specifying a supply ratio between raw
materials for forming the carrier diffusion suppressing layer. The carrier
diffusion suppressing layer may be replaced with a multi-layer
superlattice structure in which carrier diffusion layers and normal layers
each being similar to one of the cladding layers are alternately
laminated. In this case, the carrier diffusion layers are grown by
specifying a supply ratio between raw materials under a condition capable
of reducing point detects and the normal layers each being similar to one
of the cladding layers are grown under a usual condition.
According to the AlGaInP based semiconductor light emitting device of the
present invention, since the concentration of carriers in the cladding
layer is increased, it is possible to improve the temperature
characteristic, and since diffusion of the carriers into the active layer
is suppressed, it is possible to prevent degradation in static
characteristics such as operational current and shortening in service life
.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic sectional view showing a first example of a
semiconductor light emitting device of the present invention;
FIG. 2 is a schematic sectional view showing a second example of the
semiconductor light emitting device of the present invention; and
FIG. 3 is a schematic sectional view showing a related art semiconductor
light emitting device.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Hereinafter, one preferred embodiment of the present invention will be
described with reference to the accompanying drawings.
First, there will be described diffusion of atoms as a dopant. For example,
in growth of AlGaInP as a III-V compound, atoms of a group II element such
as zinc, magnesium, cadmium or beryllium are used as a p-type dopant. When
these atoms as the p-type dopant are supplied at a large atomic
concentration of, for example, about 5.times.10.sup.19 cm.sup.-3 upon
growth of AlGaInP, part of the atoms may remain inactive, that is, not
acting as carriers, or entrapped in AlGaInP as interstitial atoms.
Besides, in growth of a light emitting element, to form a cladding layer or
the like other than a layer having a high carrier concentration such as a
contact layer, atoms of a raw material are supplied at a carrier
concentration of about 1.times.10.sup.17 -5.times.10.sup.18 cm.sup.-3.
Accordingly, in some cases, inactive carriers are present; however, atoms
as a dopant are generally substituted at lattice points. For example,
atoms of the above-described group II element are substituted at the site
of the group III element as an acceptor.
Similarly, for a III-V compound semiconductor, atoms of a group VI element
such as selenium, sulfur, or tellurium are substituted at the site of a
group V element as a donor; and atoms of a group IV element such as
silicon, germanium or carbon are substituted at the site of a group III
element as a donor, but they can be substituted at the site of the group V
element as an acceptor. The above atoms of a group IV element are called
an amphoteric impurity.
These atoms are diffused, on the basis of a diffusion coefficient, from a
portion having a dense concentration to a portion having a thin
concentration through defects by the effect of heat generation,
current-carrying or strain. The defects have been regarded as
dislocations; however, in the recent year, with the developed growth
technology, it is found that the defects are mainly represented by point
defects such as atomic vacancies. To reduce occurrence of these point
defects, a perfect crystal may be grown under a growth condition of
reducing point defects due to, for example, non-stoichiometric
composition.
In growth of a III-V compound, since a vapor pressure for a group V element
is generally high with the exception depending on the growth process,
atoms of the group V element are liable to be partially eliminated from
the grown crystal. For this reason, atoms of the group V element are
supplied in a large amount as compared with atoms of a group III element,
and accordingly, the grown crystal is slightly rich in atoms of the group
V element.
In a III-V compound semiconductor, however, atoms of a group II element as
p-type carriers tend to be easily diffused because a large number of
vacancies of atoms of a group III element occur under this growth
condition. As a result, from the viewpoint of only diffusion of carriers,
by setting a V/III ratio, which is a supply ratio between raw materials of
the group V element and the group III element, at a value smaller than
that used for the usual growth condition, diffusion of the carriers can be
made smaller.
Beside, growth of a carrier diffusion suppressing layer to a large
thickness increases atomic vacancies of a group V element, and thereby it
is inconvenient from the viewpoint of the perfect crystal. To cope with
such an inconvenience, there may be formed a multi-layer structure in
which thin carrier diffusion suppressing layers and thin layers grown
under a usual growth condition are repeatedly laminated. The repeatedly
laminated layers are the same in material but are different in crystal
structure, to thus form a supperlattice structure.
The present invention will be more clearly understood with reference to the
following examples in each of which the present invention is applied to a
III-V compound semiconductor light emitting device, particularly, an
AlGaInP based semiconductor laser.
EXAMPLE 1
FIG. 1 is a sectional view of a semiconductor laser as a first example of
the present invention. The semiconductor laser is fabricated by MOCVD. In
this example, on a substrate 1 of a first conducting type, for example, on
an n-type GaAs substrate 1 are sequentially grown an n-type (first
conducting type) GaAs buffer layer 2 having a thickness of 0.3 .mu.m; an
n-type AlGaInP cladding layer 3 having a thickness of 1 .mu.m; and an
active layer 4 having a MQW (Multi-Quantum Well) structure of
GaInP/AlGaInP.
Then, a carrier diffusion suppressing layer 9 having a thickness of 50 nm
is grown on the active layer 4 at a reduced V/III ratio. For example,
while the V/III ratio for the usual cladding layer is set at 200, the
V/III ratio for the carrier diffusion suppressing layer 9 is set at 100.
The growth condition will be described in detail below. In the case of
forming the usual cladding layer, that is, in the case where the V/III
ratio is set at 200, a flow rate of PH.sub.3 as a source gas for
phosphorus as a group V element is set at 300 cc/min; a flow rate of TMG
(Trimethylgallium) as a source gas for gallium as a group III element is
set at 6 cc/min; a flow rate of TMA (Trimethylaluminium) as a source gas
for aluminum as a group III element is set at 30 cc/min; and a flow rate
of TMI (Trimethylindium) as a source gas for indium as a group III element
is set at 350 cc/min. That is, a mol ratio in flow rate between the group
V element and the group III elements becomes 200:1. On the contrary, in
the case where the V/III ratio is set at 100, that is, in the case of
forming the carrier diffusion suppressing layer, the amount of PH.sub.3
may be reduced to half, that is, 150 cc/min.
On the carrier diffusion suppressing layer 9 are sequentially grown a
p-type (second conducting type) AlGaInP cladding layer 5 having a
thickness of 1 .mu.m; a p-type GaInP layer 6 having a thickness of 0.1
.mu.m; and a p-type GaAs current cap layer 7 having a thickness of 0.3
.mu.m.
The p-type AlGaInP cladding layer 5, p-type GaInP layer 6, and p-type GaAs
current cap layer 7 are then selectively etched by typically
photolithography, to form a mesa structure. Thereafter, an n-type GaAs
current block layer 8 is grown to be laminated on both sides of the mesa
structure, to form a semiconductor light emitting device 100.
EXAMPLE 2
FIG. 2 is a sectional view of a semiconductor laser as a second example of
the present invention. The semiconductor laser is fabricated by MOCVD. In
this example, on a substrate 1 of a first conducting type, for example, on
an n-type GaAs substrate 1 are sequentially grown an n-type (first
conducting type) GaAs buffer layer 2 having a thickness of 0.3 .mu.m; an
n-type AlGaInP cladding layer 3 having a thickness of 1 .mu.m; and an
active layer 4 having a MQW (Multi-Quantum Well) structure of
GaInP/AlGaInP.
Then, a multi-layer structure for suppressing diffusion of carriers is
formed on the active layer 4. To be more specifically, the multi-layer
structure is formed by alternatively laminating carrier diffusion
suppressing layers 10 (thickness: 5 nm for each layer) grown with a
reduced V/III ratio and p-type AlGaInP layers 11 (thickness: 5 nm for each
layer) grown with a usual V/III ratio.
On the carrier diffusion suppressing layer 10 are sequentially grown a
p-type (second conducting type) AlGaInP cladding layer 5 having a
thickness of 1 .mu.m; a p-type GaInP layer 6 having a thickness of 0.1
.mu.m; and a p-type GaAs current cap layer 7 having a thickness of 0.3
.mu.m.
The p-type AlGaInP cladding layer 5, p-type GaInP layer 6, and p-type GaAs
current cap layer 7 are then selectively etched by typically
photolithography, to form a mesa structure. Thereafter, an n-type GaAs
current block layer 8 is grown to be laminated on both sides of the mesa
structure, to form a semiconductor light emitting device 110.
Each of the carrier diffusion suppressing layer 9 in Example 1 and the
carrier diffusion suppressing layer 10 in Example 2 is undoped; however,
it may be doped at a concentration of carriers lower than that for the
cladding layer.
Each of the carrier diffusion suppressing layers 9 and 10 is formed on the
side of the p-type AlGaInP cladding layer 5 of the second conducting type,
it may be formed on the side of the n-type AlGaInP cladding layer 3 of the
first conducting type with the active layer 4 put therebetween.
In Examples 1 and 2, each of the carrier diffusion suppressing layers 9 and
10 is provided at a portion adjacent to the active layer 4; however, it
may be provided at a portion separated a distance of, for example, about
30 nm from the active layer 4.
In Examples 1 and 2, the first conducting type is taken as n-type and the
second conducting type is taken as p-type; however, the first conducting
type may be taken as p-type and the second conducting type be taken as an
n-type.
Although the present invention has been described using Examples 1 and 2,
such description is for illustrative purposes only, and it is to be
understood that many changes and variations may be made without departing
from the spirit or scope of the following claims.
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